If we have heard it once we have heard it a dozen times or more in recent months: a tropical weather system known as ENSO, or the El Niño Southern Oscillation, is expected to develop in the equatorial Pacific this fall or winter, bringing more beneficial rain to the American Southwest.
But wait. It has been raining across the Southwest in recent weeks, and depending on where you are located, it has rained a lot. Taking New Mexico as an example, over the last week heavy rains and serious flooding have occurred across large areas of the state.
In fact, the National Weather Service in Albuquerque reports the southern third of New Mexico has seen 200 percent or more of its average monthly rainfall in September so far and in July and August the state received at or just above its average rainfall amount, something that hasn't happened in over four years.
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While much of New Mexico still remains in one stage of drought or another, there is no argument significant improvement has occurred in broad areas of state over the last three months to help curb the intensity of drought conditions
The same could be said for parts of Arizona and Nevada where record rainfalls have been reported in recent weeks. Texas, still suffering pockets of extreme drought, has also improved with some areas receiving rainfall over the last 90 days equal to their annual rainfall average.
If the effects of an El Niño are not already in place, then surely something like it is happening to bring such abundant rains in such a short period of time.
Professor Adam Sobel, Department of Applied Physics and Applied Mathematics & Department of Earth and Environmental Sciences at Columbia University, says while it appears as though an El Niño is influencing rain development across the Southwest, what we are actually seeing is a related weather phenomenon that is giving the same benefit and creating many of the same conditions, but which is not yet a full-blown ENSO event, which may still be coming in the weeks ahead.
El Niño? Maybe not yet
"As of late August, tropical atmospheric temperatures appear to be responding more strongly to the ocean than they typically do at this early stage of El Niño development. This may be precisely because the tropical ocean as a whole, rather than just the eastern Pacific, is already warm," Sobel said last week in a written report on the NOAA Climate.gov website.
He explained that the temperature of the atmosphere is about the same everywhere in the tropics, as long as you look only at a fixed altitude or pressure level. At the same time, the ocean surface can sustain somewhat larger temperature gradients than the atmosphere can.
"So, in general, a tropical atmosphere with the same temperature—again at a fixed level—sits over a tropical ocean with a wider range of temperatures."
But the temperatures of the atmosphere and ocean are linked because they exchange heat and water vapor. That heat exchange occurs at the air-sea interface and is carried up into the atmosphere by tall, rain-producing clouds. So neither the ocean surface nor the atmosphere can change its temperature without influencing the other.
If the ocean gets a little warmer relative to the atmosphere, the atmosphere just above the sea surface gets more warm and humid, and that warm, humid air becomes buoyant and rises in clouds. The heating that occurs in the clouds when water vapor condenses is then distributed throughout the atmosphere by large-scale circulations, which maintain horizontal uniformity of temperature within the tropics.
At the same time, the clouds will shade the sea surface from the Sun, cooling it, and the storm circulations generated by the clouds will bring cool, gusty downdrafts, which will cool the surface further. If the ocean gets a little cooler relative to the atmosphere, the process works in reverse: clouds will be reduced because the cooler and drier air at the surface will be less buoyant, and more sunlight and a calmer surface will allow the surface to warm up.
The report adds that in an ENSO event, the SST anomalies in the central and eastern Pacific are often generated by ocean dynamics. However, in the early stages of an El Niño, the surface warmth in the eastern and central Pacific—at least those portions already warm enough to support convective clouds—is carried up into the atmosphere by those clouds.
That warms the atmosphere above the SST anomalies. Because the tropical atmosphere can’t sustain large horizontal differences in temperature, the heat exchange ultimately warms the rest of the tropical atmosphere as well.
So early in a typical event, the atmosphere in regions far from the eastern Pacific SST anomalies—in the Indian Ocean, western Pacific, or Atlantic for example—becomes a little warmer relative to the ocean surface than it would normally be. Essentially, the atmosphere there already knows about the building El Niño event, while the ocean does not.
Hope for wet winter
He goes on to explain that most of the tropical ocean surface—not just the eastern and central Pacific—is already warm, not even waiting for El Niño to officially arrive, and this condition could provide a clue to what is behind what appears to be the arrival of an El Niño but is little more than the movement of moisture-laden air over an unstable atmosphere hanging above the American Southwest.
But it raises a good question and one that many are beginning to ask: "If the current and recent rains are not being caused by an ENSO, then is rain, and how much, coming when El Niño actually arrives later this month or in October?" Could this be the first fall and winter in a long time that abundant rains prepare the way for a new growing season in 2015, or is it still too early to tell what evolving weather will do in the weeks and months ahead?
There seems to be hope among some Southwest farmers and ranchers who say they believe El Niño is indeed stirring on the far horizon and fall and winter rains will come, a perfect addition to the moisture already received in recent weeks. If that happens, they see a good chance for deep soil moisture recovery, at least in some parts of the Southwest.
It may be too early to count chickens, but there's nothing wrong with exercising a little guarded optimism when it comes to the prospect of a wet winter.